I guess I could add a few points. My experiences are similar to what cowboy posted above, although I have never flown anything but economy class for international flights. I also never use miles for a class upgrade.
I tend to have more FF miles in my United account than any other because United usually works for me when I fly domestically (in the U.S.). I tend to pay for domestic flights and use FF miles for international flying. Using 25,000 miles for a domestic flight is generally not as big a savings as using 60,000 miles for an Asia flight.
When dealing with an international itinerary that includes multiple legs it’s important to remember that when you are paying for a flight your options are not as good as when you are using FF miles. This is because most airlines have FF partners. When using miles they will put you on different carriers for different legs of the trip. This can be very helpful and is another real good reason to use the FF miles for International flights. Thai Airways and Singapore Airlines are partners with United. So my United miles gives me options on these other carriers.
For the long flight over the Pacific I have flown United, Northwest, Thai Airways, and Singapore Airlines. Quality-wise, United is better than NW, Thai is better than United, and Singapore is better than Thai.
Since Thai, Sing Air and United are partners - if using United FF miles you have the option of the Singapore Air flight to HK that leaves out of SFO at midnight, and then taking Thai Air from HK to BKK. I don’t think you have that option if you are buying the ticket unless you buy each leg separately (which would be foolish). I like that option because I am more apt to sleep on that redeye flight, and I arrive at BKK in the morning instead of late at night. I also get to fly Singapore Air which is nice.
If buying the ticket I go primarily by price but will pay up a little to fly Thai Air. I have never found a good price for Singapore Air, but maybe that’s just been my bad luck. I also have never flown Cathey because, again, I have never found a good price.
Thai has a “direct†fight between BKK and LAX. I think United also has this (at least they did a few years ago). This is misleading because there is still a stop (Thai stops in Japan and United stopped in HK), and at that stop you must deplane and pass through another customs check. As far as I am concerned this is just no different than having to make a standard stop and change planes.
My experience has also been that, as of the past few years, the online travel agencies (Orbitz, Travelocity, etc.) NEVER have better prices than you can get by going directly to the website of the airlines. For all tickets, domestic or international, I go to Travelocity and do a search just to find out which airlines travel to my destination. Then I go to the airline’s website for prices (being sure to check all their promotions). Perhaps others have a different opinion on this because I see people recommending online travel agencies – but this just never works for me. I always check the travel agencies and have not bought from any of them in over four years.
At the risk of rambling, Priceline and Hotwire never work for me for airline tickets, period. Priceline DOES work for U.S. hotels (never tried for out of the country) – however you must know exactly how to bid and that’s another long story.
Hope any of the above was helpful to someone.